Interstate 81 briefly enters the eastern panhandle of West Virginia, serving the city of Martinsburg. It then crosses the state of Maryland at one of its narrowest points, passing through the "Hub City" of Hagerstown where it intersects with a large number of other routes.

Interstate 81 forms a major north–south corridor through the state of Pennsylvania, serving the boroughs of Chambersburg and Carlisle, where it meets the Pennsylvania Turnpike. But I-81 does not directly interchange with it (motorists must use US 11 to connect). Around the state capital of Harrisburg, the route forms the northeastern section of Pennsylvania's Capital Beltway. The route then travels northeast toward the Wyoming Valley, where it serves the cities of Wilkes-Barre and Scranton, then heads north through the Endless Mountains region toward the state line. Although it is signed as a north–south highway, it is distinctly northeast–southwest and occasionally east–west throughout the state.

The Interstate 81 Corridor Coalition, a six-state coalition, was organized to handle issues along Interstate 81, such as truck traffic and air pollution. As of October 15, 2008, this commission has met twice.[3][4]